Louis Pasteur

Fermentation and germ theory of diseases
Pasteur was motivated to investigate fermentation while working at Lille. In 1856 a local wine manufacturer, M. Bigot, whose son was one of Pasteur's students, sought for his advice on the problems of making beetroot alcohol and souring.^[54][7]
According to his son-in-law, René Vallery-Radot, in August 1857 Pasteur sent a paper about lactic acid fermentation to the Société des Sciences de Lille, but the paper was read three months later.^[55] A memoire was subsequently published on 30 November 1857.^[56] In the memoir, he developed his ideas stating that: "I intend to establish that, just as there is an alcoholic ferment, the yeast of beer, which is found everywhere that sugar is decomposed into alcohol and carbonic acid, so also there is a particular ferment, a lactic yeast, always present when sugar becomes lactic acid."^[57]
Pasteur also wrote about alcoholic fermentation.^[58] It was published in full form in 1858.^[59][60] Jöns Jacob Berzelius and Justus von Liebig had proposed the theory that fermentation was caused by decomposition. Pasteur demonstrated that this theory was incorrect, and that yeast was responsible for fermentation to produce alcohol from sugar.^[61] He also demonstrated that, when a different microorganism contaminated the wine, lactic acid was produced, making the wine sour.^[7] In 1861, Pasteur observed that less sugar fermented per part of yeast when the yeast was exposed to air.^[61] The lower rate of fermentation aerobically became known as the Pasteur effect.^[62]

Pasteur experimenting in his laboratory.

Institut Pasteur de Lille
Pasteur's research also showed that the growth of micro-organisms was responsible for spoiling beverages, such as beer, wine and milk. With this established, he invented a process in which liquids such as milk were heated to a temperature between 60 and 100 °C.^[63] This killed most bacteria and moulds already present within them. Pasteur and Claude Bernard completed tests on blood and urine on 20 April 1862.^[64] Pasteur patented the process, to fight the "diseases" of wine, in 1865.^[63] The method became known as pasteurization, and was soon applied to beer and milk.^[65]
Beverage contamination led Pasteur to the idea that micro-organisms infecting animals and humans cause disease. He proposed preventing the entry of micro-organisms into the human body, leading Joseph Lister to develop antiseptic methods in surgery.^[66]
In 1866, Pasteur published Etudes sur le Vin, about the diseases of wine, and he published Etudes sur la Bière in 1876, concerning the diseases of beer.^[61]
In the early 19th century, Agostino Bassi had shown that muscardine was caused by a fungus that infected silkworms.^[67] Since 1853, two diseases called pébrine and flacherie had been infecting great numbers of silkworms in southern France, and by 1865 they were causing huge losses to farmers. In 1865, Pasteur went to Alès and worked for five years until 1870.^[68][69]
Silkworms with pébrine were covered in corpuscles. In the first three years, Pasteur thought that the corpuscles were a symptom of the disease. In 1870, he concluded that the corpuscles were the cause of pébrine (it is now known that the cause is a microsporidian).^[67] Pasteur also showed that the disease was hereditary.^[70] Pasteur developed a system to prevent pébrine: after the female moths laid their eggs, the moths were turned into a pulp. The pulp was examined with a microscope, and if corpuscles were observed, the eggs were destroyed.^[71][70] Pasteur concluded that bacteria caused flacherie. The primary cause is currently thought to be viruses.^[67] The spread of flacherie could be accidental or hereditary. Hygiene could be used to prevent accidental flacherie. Moths whose digestive cavities did not contain the microorganisms causing flacherie were used to lay eggs, preventing hereditary flacherie.^[72]